AM processes generally involve the buildup of one or more materials to make a net or near net shape (NNS) object, in contrast to subtractive manufacturing methods. Though “additive manufacturing” is an industry standard term (ASTM F2792), AM encompasses various manufacturing and prototyping techniques known under a variety of names, including freeform fabrication, 3D printing, rapid prototyping/tooling, etc. AM techniques are capable of fabricating complex components from a wide variety of materials. Generally, a freestanding object can be fabricated from a computer aided design (CAD) model. A particular type of AM process uses electromagnetic radiation such as a laser beam, to solidify a photopolymer, creating a solid three-dimensional object.
FIG. 1 is schematic diagram showing a perspective view of an exemplary conventional apparatus 100 for additive manufacturing. The apparatus 100 uses selective laser activation (SLA) such as disclosed in U.S. Pat. No. 5,256,340, assigned to 3D Systems, Inc. to form a part 130 as a series of layers. The apparatus 100 includes a vat 110 that holds a liquid photopolymer 112, which may also be referred to as a resin. A build plate 116 is oriented in an x-y plane and forms the base upon which the part 130 is formed. An elevator 114 moves the build plate 116 along a z-axis orthogonal to the x-y plane. A sweeper 118, spreads the liquid photopolymer 112 across the build plate 116 and previously solidified layers of the part 130.
A laser 120 provides a laser beam 126 that solidifies the liquid photopolymer 112 according to a curing depth, which generally corresponds to a layer thickness. Lenses 122 adjust properties of the laser beam 126 such as beam width. A scanning mirror 124 reflects the laser beam 126 at various angles to scan a pattern in a top layer of the liquid photopolymer 112. The apparatus 100 is under the control of a computer 140 that directs the scanning mirror 124 as well as the elevator 118 and laser 120. The computer controls the apparatus 100 such that the laser 120 solidifies a scan pattern in the top layer of the liquid photopolymer 112. The elevator 114 then moves the build plate 116 downward along the z-axis and the sweeper 118 spreads the liquid photopolymer 112 to form a new top layer above the previously solidified photopolymer. The process continues layer by layer until the part 130 is formed on the build plate 116.
Various additive manufacturing apparatuses operate on a slice-based modelling technique. For example, as described in U.S. Pat. No. 5,184,307, a stereolithography system will typically form a three-dimensional part in accordance with a corresponding object representation, which representation may be formed in a CAD system or the like. Before such a representation can be used, however, it must be sliced into a plurality of layer representations. The stereolithography system will then, in the course of building up the object in a stepwise layer-by-layer manner, selectively expose the untransformed layers of material in accordance with the layer representations to form the object layers, and thus, the object itself.
When exposing the untransformed layers of material, the scanning mirror 124 traces a pattern in the layer. Generally, the stereolithography system will control the scanning mirror 124 to trace the outline of any shapes in the layer, then fill the shape in with a series of hatch lines. The present inventors have discovered that various shapes such as thin walls do not necessarily form as solid parts. Instead, the shape may include unsolidified liquid photopolymer within the part. Generally, stereolithography systems are designed for rapid prototyping to illustrate a design concept. Such uses do not require consistent material properties or strict manufacturing tolerances. Accordingly, unsolidified liquid photopolymer may be acceptable for rapid prototyping. When stereolithography is used in commercial manufacturing, however, unsolidified liquid photopolymer within a part results in poor material properties (e.g., structural weakness) and the parts do not satisfy manufacturing tolerances.
One solution to unsolidified liquid photopolymer is to increase the power of the laser 120. The increased power, however, causes greater cure depths in the z-dimension in some portions of the part. This
The present inventors have further discovered that the unsolidified liquid photopolymer within the part is due to draw speeds. Generally, available additive manufacturing apparatuses set the draw speed to finish parts as quickly as possible. Available additive manufacturing apparatuses do not allow user control of the draw speed.
In view of the above, it can be appreciated that there are problems, shortcomings or disadvantages associated with AM techniques, and that it would be desirable if improved methods of forming thin walled structures were available.